Engines are constantly being modified to increase power output. The increase in power output of engines often subjects piston assemblies to higher combustion chamber pressures and temperatures. It is for that reason that emphasis has been placed over the past several years on finding new and economical designs for manufacturing a piston assembly capable of withstanding these higher combustion chamber pressures and temperatures.
The goal of the prior art piston assemblies was to provide economical piston assemblies by reducing excessive weight and limiting structural components. The prior art piston assemblies included a two piece piston assembly with a wrist pin attached to an upper piston member, resulting in a simplified skirt or crown geometry, and improved manufacturability. However, the wrist pin is attached to the piston member by extending a bolt through the wrist pin and into the upper piston member on opposing sides of the connecting rod. Due to the high combustion pressures experienced during operation of present day engines, the bolting pattern and loading design of the prior art allows the wrist pin to flex about the central axis of the piston assembly causing failure of the design. Required bolt access on the piston underside also limits the connecting rod bearing length, and thereby, piston load capacity.
Several problems, one of which is the height of piston assemblies may increase the overall engine height and cost of the engine. Engines that operate using a piston assembly usually have an upper piston member made of a material, such as cast iron or steel that can withstand the thermal and mechanical stresses that occur during engine operation. The piston skirt is usually made of lighter weight material, such as aluminum or an aluminum alloy to help reduce the weight of the piston assembly. Connecting the piston skirt to the piston crown is typically accomplished using threaded fasteners. Stiffness of the piston assembly, in particular, at the bearing surface and around the fastener is characteristic of the material of the upper piston member and piston skirt. Stiffness is a function of the materials modulus of elasticity, length, and cross sectional area. Piston skirts made of lighter materials may require a greater piston skirt height in order to provide an adequate stiffness ratio between the bearing surface and the fastener and to maintain proper operational characteristics of the piston assembly. The increased piston skirt height leads to an undesirable increase in the height of the engine block, cylinder liner, and other components. This results in an increase in the cost of the piston assembly and related engine components increase in manufacturing cost is due, in particular, the engine block that needs to be lengthened to accommodate the piston assembly.
Wear often occurs at the joint interface between the upper piston member and piston skirt at the outer portion of the bearing surface and around the fasteners. Bearing surface wear is due to radial and axial movements of the bearing surface. Fastener wear is due to radial and axial movements of the bearing surface of the upper piston member and piston skirt, which surfaces contact each other. These movements are caused by the warping action caused by the deformation of the upper part of the piston owing to the combustion chamber pressure and resulting not only in wear but also the risk of a plastic deformation of the piston skirt with a subsequent cracking and shearing of material. Additionally, the plastic deformation causes the upper piston member and piston skirt to approach each other so that the tension of the fasteners are reduced.
The present invention is directed to overcoming one or more of the problems set forth above.